Microorganisms may be cultured in an aqueous medium to produce a variety of products such as lipids, proteins, pigments, and polysaccharides which may be used in the production of food, feed, fuel, pharmaceuticals, nutraceuticals, fertilizers, cosmetics, and plastics. Multiple bioreactor designs are capable of culturing microorganisms in an aqueous medium, including opened and closed bioreactor systems. A closed bioreactor system provides several advantages over an open bioreactor system, such as: the ability to shield the culture of microorganisms from outside contamination, the ability to limit water loss through evaporation, and the ability to better control the exchange of gasses between the microorganisms and the aqueous medium. The increased control provided by a closed bioreactor system also facilitates the ability to reproduce the quality of the harvested microorganisms, and meet requirements for containing genetically modified organisms. Closed bioreactor systems may comprise tubular, tank, bag, and panel bioreactors.
Tubular bioreactors may continuously circulate an aqueous culture in a single or plurality of flow paths provided by tubes in a straight, serpentine, helical, winding, or spiral arrangement. Tubular bioreactors have commonly been used for growth of microorganisms in phototrophic culture conditions, but tubular bioreactors may still experience drawbacks such as stagnation zones in the flow path, biofouling on the inner surface of the tubes and associated elbows or the de-gas tank, and inefficient engineering design which hampers reconfiguration or repair of the bioreactor system. Additionally, with the added complexity resulting in culture conditions comprising an organic carbon source, such as increased growth rate and production of gases in the culture, conventional tubular bioreactors used in phototrophic culture conditions are not well equipped to facilitate growth in mixotrophic or heterotrophic culture conditions, or switch between different culture conditions. Further, optimized designs for mixotrophic or heterotrophic systems may use tube diameters larger than those typically employed for phototrophic systems where short light path is needed to facilitate growth by availability of light. Optimized tube diameters for mixotrophic or heterotrophic systems may be larger than 10 cm and range from 10 to 100 cm. Tube diameters may range from 2 to 200 cm, with a preferred range for a phototrophic only system from 2 to 10 cm.
Tubular bioreactors known in the art are not designed optimally for commercial production. For example, engineering design of a conventional tubular bioreactor system is typically a single integrated system, which results in the entire tubular bioreactor system being unusable when a single component is not functional. Even when a non-functioning part may be isolated, the integrated system design is not optimal for upgrading or repairing the system. Also, the conventional single integrated tubular bioreactor system design is restricted to one configuration or set up, and does not have the flexibility to adapt to different configurations for different culture volumes or condition requirements. Also, material selection in a convention tubular bioreactor system, such as polyvinyl chloride (PVC) tubes, may result in unnecessary stagnation zones near connections, surface finish aggravated biofouling, or material degradation, causing suboptimal mixing or contamination of the microorganism culture.
In another example of sub-optimal design, a conventional tubular bioreactor system may space the tube segments for light application, but do not strategically configure the tube segments and lighting devices for optimal delivery of light to the aqueous culture of microorganisms within the tube and minimal light energy going unutilized by the microorganisms. Sub-optimal configuration of the tube segments may result in tube segments shading the aqueous culture disposed in other tube segments from light at certain locations, or a sub-optimal light path from the lighting source to the microorganisms. Additionally, sub-optimal positioning of lights can result in wasted light energy, which may be lighting more than the culture within the bioreactor tubes. Also, using conventional lighting systems may result in the application of harmful amounts of light or wavelengths of light which cannot be utilized by the microorganisms.
In an additional example of the limitations of conventional tubular bioreactors, a conventional tubular bioreactor engineered for phototrophic conditions may not be equipped to provide proper nutrients or gas exchange for mixotrophic and heterotrophic growth. The utilization of an organic carbon source in mixotrophic and heterotrophic cultures may increase the microorganism growth and culture density faster than a phototrophic bioreactor is equipped to handle. Also, the rate of gas saturation will differ from phototrophic cultures, as well as the oxygen and carbon dioxide production and consumption rates. A tubular bioreactor without the flexibility to accommodate various culture conditions limits the utility of the bioreactor for product production.
Therefore, there is a need in the art for a closed bioreactor system which is optimized for performance and provides the flexibility to accommodate different culture conditions.